Field of the Invention:
The present invention relates to a process for producing a pressure-sensitive electroconductive sheet which is used as a switch element for electronic machines and equipment. More particularly, it is concerned with a process for producing a pressure-sensitive electroconductive sheet, with the switching conductor circuits and electrodes contained in the sheet, the sheet having superior conductivity, resolution, and durability.
Description of the Prior Art:
Pressure-sensitive conductive rubber sheets have been in general use as a switching element (member) for the keyboard of electronic machines and equipment. Such sheets are produced by dispersing metal particles into insulating rubber. If the degree of dispersion is properly controlled, the metal particles come into contact with one another upon application of a threshold pressure forming conductive paths and making the rubber electrically conductive. Otherwise, the rubber retains its high resistance as an insulator when it is not under pressure.
The conventional pressure-sensitive conductive rubber sheet is produced by several processes. According to the process disclosed in Japanese Patent Publication Kokai No. 52-73399, conductive metal particles previously surface-treated with a rubberlike insulating binder are incorporated and dispersed into a polymeric elastomer, and the filled elastomer is formed into a sheet. According to the process disclosed in Japanese Patent Publication Kokai No. 52-125796, conductive particles previously treated with a silane coupling agent are dispersed into a polymeric elastomer and the filled elastomer is formed into a sheet. According to the process disclosed in Japanese Patent Publication Kokai No. 53-897, magnetic metal particles are dispersed into an elastomer and the filled elastomer is formed into a sheet in a parallel magnetic field so that the magnetic metal particles are regularly arranged. Thus, the pressure-sensitive conductive rubber sheet produced by the above process varies greatly in resistance when subjected to pressure and it also works with a low pressure.
In the case of conventional pressure-sensitive conductive sheet, silicone rubber is used as the binder into which conductive particles are dispersed. This is because silicone rubber has good resilience and environmental resistance. On the other hand, silicone rubber is poor in mechanical strength, and this shortcoming leads to the poor durability of pressure-sensitive conductive rubber sheet. At ordinary levels of mechanical strength, the conventional pressure-sensitive conductive rubber sheet is required to have a thickness of 0.5 to 1 mm. Such a thick sheet is not suitable as a switch element for electronic parts which are becoming lighter and thinner.
Another disadvantage of the conventional pressure-sensitive electroconductive sheet is that the sheet has to be cut into a proper shape according to the circuit electrode before it is used as a switch element for electronic parts. This cutting step is inconvenient and difficult.